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The Berkeley remote
login command (rlogin) allows a user to
easily access a remote UNIX host over a TCP/IP internetwork. A user
would normally use rlogin when he or she needs to log in to a
server to perform a number of tasks. There are some situations, however,
where a user needs to only enter one command on a remote host. With
rlogin, the user would have to log in to the host, execute the
command, and then log back out again.

This isnt exactly an earth-shattering
amount of inconvenience, especially when the correct configuration files
are set up to allow automatic login. Over the course of time, however,
all the extra logging in and out can become tedious. As a convenience,
a variation of rlogin was created called remote shell (rsh),
which allows a user to access a remote host and execute a single command
upon it without requiring the login and logout steps.

Note: Shell is the standard term used in UNIX to refer to the user interface that accepts commands from the user and displays output on the screen.

rsh Operation

The rsh command is based on
rlogin and really works in much the same way, except that it
is oriented around executing a command rather than establishing a persistent
login session. The server process on the remote host is usually called
rshd (remote shell daemon) and listens for incoming rsh
requests. When one is received, the user is logged in through the same
mechanism as rlogin. The command is then run on the remote host
and the user is automatically logged out.

Automated Use of rsh by Programs

rsh is most useful when automatic
login is employed, so that the program can be run without the need for
the user to enter a login name or password. In that case, it is possible
to have programs use rsh to automatically run commands on remote
hosts without the need for human intervention, which opens up a number
of possibilities for UNIX users. The normal UNIX user interface concepts
of standard input (stdin), standard output (stdout) and
standard error (stderr) also apply to rsh, so you can use it
to execute a remote command and redirect the output to a local file.
For example, this command would let a user get a listing of his or her
home directory on the server <somehost> and store
it in the local file remotelist:

rsh <somehost> ls
-l >remotelist

Key Concept: The remote shell or rsh command is similar to the Berkeley rlogin command, but instead of opening a login session on a remote host, it executes a single, user-provided command. rsh can be helpful for users who need to perform a quick operation on a remote host, and can also be employed by other programs to automate network tasks.

rsh Security Concerns and ssh

Since rsh is based on rlogin,
all of the concerns that apply to rlogin are also relevant here,
especially with regard to security. (We no more want an unauthorized
user running commands on our servers than logging in to them!) As with
rlogin, newer versions of rsh support more advanced authentication
options than the original software. Also, just as slogin is a
newer, more secure version of rlogin, there is a program called
ssh (secure shell) that replaces rsh on many systems.

Note: On some systems, if rsh is entered without a command specified to execute, then an interactive remote session is established, exactly as if the rlogin command had been entered instead of rsh.

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